Add Streamable HTTP transport support (MCP spec 2025-11-25)

[alexei-led]

Summary

MCP spec revision 2025-11-25 replaced the HTTP+SSE transport with Streamable HTTP. Our server currently supports stdio and sse transports — sse is now legacy (spec 2024-11-05).

Spec Reference

• Streamable HTTP transport spec
• Changelog — SSE replaced by Streamable HTTP since 2025-03-26

Current State

File: src/aws_mcp_server/__main__.py (lines 37-49)
File: src/aws_mcp_server/config.py (line 22)

# config.py
TRANSPORT = os.environ.get("AWS_MCP_TRANSPORT", "stdio")

# __main__.py  
if TRANSPORT not in ("stdio", "sse"):
    logger.error(f"Invalid transport protocol: {TRANSPORT}. Must be 'stdio' or 'sse'")
    sys.exit(1)
# ...
mcp.run(transport=TRANSPORT)

Required Changes

1. Add streamable-http as a valid transport — update validation in __main__.py to accept "stdio", "sse", and "streamable-http"
2. Make streamable-http the recommended HTTP transport — update README/docs to recommend streamable-http over sse
3. Deprecate sse transport — add deprecation warning when sse is selected, suggest migration to streamable-http
4. Update config.py — add streamable-http to the docstring for AWS_MCP_TRANSPORT
5. Verify FastMCP support — fastmcp>=2.13.0 should support streamable-http natively via mcp.run(transport="streamable-http"). Confirm and bump minimum version if needed.

Streamable HTTP Key Requirements (from spec)

• Server provides a single HTTP endpoint supporting POST and GET
• POST for client→server JSON-RPC messages
• GET for server→client SSE stream (optional, for notifications)
• Must validate Origin header (return 403 for invalid)
• Bind to 127.0.0.1 locally (already handled for SSE via is_docker_environment())
• Session management via Mcp-Session-Id header

Migration Notes

The sse transport should remain functional for backward compatibility but emit a deprecation warning. Example:

if TRANSPORT == "sse":
    logger.warning(
        "SSE transport is deprecated per MCP spec 2025-11-25. "
        "Use 'streamable-http' instead. Set AWS_MCP_TRANSPORT=streamable-http"
    )

Testing

• Verify streamable-http transport starts correctly
• Verify SSE still works with deprecation warning
• Test Docker environment binding (0.0.0.0 vs 127.0.0.1)
• Integration test with MCP client (e.g., Claude Desktop)